1. Field of the Invention
The present invention relates generally to a heat transfer tube for a heat exchanger, and more particularly to a heat transfer tube with cross-grooved inner surface in order to improve the fluidity and the heat transfer characteristic thereof.
2. Description of the Prior Art
As heat exchangers, such as vaporizing tubes, condensing tubes or heat pipes, for use in air conditioners, refrigerators or the like, to evaporate or condense the refrigerant flowing inside the tube by heat transferring with fluids flowing outside the tubes, internally grooved heat transfer tubes have mainly been used from the standpoint of attaining high efficiency and energy saving.
Because fine triangular or trapezoid grooves are formed spirally in the inner surface of the tubes, the flow of refrigerants along the longitudinal direction of the tubes is promoted by turbulent flows due to the surface tension effects and the spiral grooves of the tubes. When used in the condensers, these heat transfer tubes produce superior turbulent flow of the refrigerant to improve the condensation characteristic, because a ridge formed between grooves serves as a condensing nucleus. Otherwise, when used in the evaporators, the vaporizing characteristic of the refrigerant supplied into the heat transfer tube is improved with the stirring action occurring at the edges of the grooves, in which the edge of the groove serves as a vaporizing nucleus.
U.S. Pat. No. 4,658,892 issued to Shinohara et al., ("Shinohara") on Apr. 21, 1987 discloses a heat transfer tube having relatively deeper grooves on the inner surface of the tube within a range in which the pressure loss of fluid inside of grooved tube is not substantially increased. According to Shinohara, the ratio Hf/Di of the depth Hf of the grooves to the diameter Di of the inner surface of the tube is 0.02 to 0.03, and the helix angle .beta. of the grooves to an axis of the tube is 7.degree. to 30.degree.. The ratio S/Hf of the cross-sectional area S of respective grooved section to the groove depth Hf ranges from 0.15 to 0.40, and the apex angle in cross-section of a ridge located between the respective grooves ranges from 30.degree. to 60.degree..
In the heat transfer tube disclosed in Shinohara, the refrigerant fluid supplied into the tube becomes more widely distributed over the entire inner surface of the tube along the continuous helix grooves, leading to deterioration of the condensation efficiency.
In order to improve the heat transfer characteristic, it have been proposed that a heat transfer tube with a number of secondary grooves intersecting the primary spiral grooves at a desired angle and spacing at a constant interval. See U.S. Pat. No. 4,733,698 issued to Sato et al. ("Sato") on Mar. 29, 1988.
For example, FIG. 9A illustrates the heat transfer tube with secondary grooves 12 intersecting first primary grooves 11 at a desired angle, in which the secondary grooves are sloped at a helix angle larger than helix angle of the first spiral grooves.
In such cross-grooved heat transfer tubes, the internal surface area increased by the secondary grooves 12 improves heat transfer efficiency. Also, due to the helix angle of the secondary grooves being larger than the helix angle of the primary grooves with respect to the axial direction of the tube, as well as the increase of the number of the edges in the tube, the stirring action for the refrigerant fluid increases. Therefore, the evaporation characteristic of the refrigerant fluid is improved, resulting in the spread of the application range, gradually.
In the conventional cross-grooved heat transfer tube, however, a current of the fluid moving against the main current and with a circular motion (hereinafter referred to as "eddy") is produced on the downstream slant face of the ridge 13 in the secondary groove 12 formed between the ridges 13, as illustrated in FIGS. 9B and 9C. The production of the eddy gives resistance to the flowing direction of the refrigerant fluid inside the tube, resulting in deterioration of heat transfer characteristic in the eddy producing area.
Also, when manufacturing the heat transfer tube described above, the first spiral grooves 11 are roll-formed, and then the secondary grooves 12 are roll-formed. Accordingly, protrusions 14 are protruded on both sides of the spiral grooves 11, which are already formed, in roll-forming the secondary grooves. The protrusions 14 formed due to the above method causes the flowing resistance to increase, thereby deteriorating the turbulent effects produced by the spiral grooves. Accordingly, although the conventional cross-grooved heat transfer tube has a superior heat transfer characteristic, such effect comes at the cost of a significant pressure loss inside the tube.
In order to overcome the problem described above, Japanese Patent Unexamined Publication No. 94-147786 discloses a heat transfer tube in which the primary grooves are formed on the tube's internal surface in the shape of a rectangle or an inverted trapezoid with a constant depth H and a constant pitch P along the longitudinal direction of the tube, and secondary grooves with a depth shallower than the primary grooves' depth are formed in a direction intersecting the primary grooves. In the primary grooves, the ratio S/P of width S of the bottom to the pitch P is below 1/2, and the ratio L/S of the depth L to the width S is above 1/2.
As described above, although the heat transfer characteristic may be improved, if the pressure loss increases, substantially increase in power is needed to let the refrigerant fluid flow in the tube. Therefore, it would be disadvantageous that the conventional heat transfer tube has the heat transfer characteristic in inverse proportion to the energy efficiency.